Stability of smectic phases in the Gay–Berne model

We present a detailed computer simulation study of the phase behavior of the Gay–Berne liquid crystal model with molecular anisotropy parameter k=4.4. According to previous investigations: (i) this model exhibits isotropic (I), smectic-A (Sm-A), and smectic-B (Sm-B) phases at low pressures, with an additional nematic (N) phase between the I and Sm-A phases at sufficiently high pressures; (ii) the range of stability of the Sm-A phase turns out to be essentially constant when varying the pressure, whereas other investigations seem to suggest a pressure-dependent Sm-A range; and (iii) the range of stability of the Sm-B phase remains unknown, as its stability with respect to the crystal phase has not been previously considered. The results reported here do show that the Sm-A phase is stable over a limited pressure range, and so it does not extend to arbitrarily low or high pressures. This is in keeping with previous investigations of the effect of molecular elongation on the phase behavior of Gay–Berne models. A detailed study of the melting transition at various pressures shows that the low-temperature crystalline phase melts into an isotropic liquid at very low pressures, and into a nematic liquid at very high pressures. At intermediate pressures, the crystal melts into a Sm-A liquid and no intermediate Sm-B phase is observed. On the basis of this and previous investigations, the reported Sm-B phase for Gay–Berne models appears to be a molecular solid rather than a smectic liquid phase.Financial support of the Spanish DGICYT (Dirección General de Investigación Científica y Técnica) under Project No. BFM-2001-1420-C02-02 is acknowledged. Additional support from Universidad de Huelva and Junta de Andalucía is also acknowledged

Molecular simulation of model liquid crystals in a strong aligning field

We report a computer simulation study of systems of perfectly aligned molecules interacting through the Gay–Berne (GB) potential model for two different values of the molecular anisotropy parameter k, namely 3 and 4.4. The models are appropriate to gauge the effects of strong aligning fields on the thermodynamics and structural properties of thermotropic liquid crystals. According to our results, one of the main effects of the external field is to increase the range of stability of the smectic A phase, which indicates the existence of a strong coupling between orientational and translational order. For the k=3 GB model the smectic phase, which is not stable in the absence of the field, is promoted when the molecules are constrained to be parallel. According to the simulation results, the smectic A-nematic transition is, in general, continuous; however, this transition appears to be first order at low pressure for the k=4.4 GB fluid model.Financial support is due to project number FIS2004- 06627-C02-01 of the Spanish Dirección General de Investigación. Additional support from Universidad de Huelva and Junta de Andalucía is also acknowledged

An examination of the vapour-liquid interface of associating fluids using a SAFT-DFT approach

With a realistic description of the free energy of bulk fluids, it is now possible to make accurate predictions at the molecular level for the phase behaviour of systems as complex as aqueous solutions of amphiphiles, reacting and associating fluids, polymers, and electrolytes (e.g. using the statistical associating fluid theory SAFT). A quantitative molecular description of the interfacial properties of inhomogeneous fluids, including surface tension and adsorption is much less common. In this work we first hope to improve the general understanding of the effect of association on the vapour-liquid interface. The vapour-liquid interface of the inhomogenous associating fluid is examined by combining the SAFT and density functional theory (DFT) approaches. A simple SAFT-HS representation is employed as it incorporates all of the essential physics of associating fluids and provides a good representation of the vapour pressure and coexisting phases. In this simplified SAFT approach the bulk fluid is represented as a hard-core reference, the association is treated with Wertheim’s first order perturbation theory (TPT1), and a van der Waals mean-®eld approximation is used for the dispersive attractive interactions. In order to keep the representation of the bulk fluid and interface at the same level of approximation we use the van der Waals theory for non-uniform fluids, which is a DFT at the level of a local density approximation (LDA); the correlations are neglected in the attractive non-local term. The vapour-liquid interface of model systems with one, two and four bonding sites are examined for varying degrees of association. As expected, a stronger site±site interaction is generally found to sharpen the interface (decrease the interfacial thick- ness) and increase the surface tension. In the case of a dimerizing (single site) fluid a limiting behaviour is reached for full association (saturation) where the molecular species are dimers. After an in depth analysis of the effect of association on the vapour-liquid interface, we highlight the strengths of our simple SAFT-DFT approach by making some quantitative comparisons with experimental surface tensions for selected systems including water and replacement refrigerants.FJB would like to thank BP Amoco Exploration and the Oil Extraction programme of the Engineering and Physical Sciences Research Council (EPSRC) for a research fellowship (GR/N20317). We acknowledge support from the Joint Research Equipment Initiative (JREI) of the EPSRC for computer hardware (GR/ M94427), and the Royal Society-Wolfson Foundation for the award of a refurbishment grant. This work was also supported by a research project from VII Plan Propio de Investigación de la Universidad de Huelva. This financial support is gratefully acknowledged